Increasingly, the drilling of oil and gas wells is no longer a matter of drilling a vertically straight bore hole from the surface to the zone of hydrocarbon recovery using a traditional drilling platform surmounted by a derrick used to support a string of jointed drill pipe having a bit at the lower end thereof. Rather, technology and techniques have been developed to deviate the bore's trajectory at angles of up to and sometimes exceeding 90.degree. from the vertical. Directional drilling offers numerous advantages including new approaches to oil and gas traps having non-conventional geometries, economic zone enhancement as can occur for example if the bore hole actually follows an oil or gas bearing strata, improved economics particularly in an over-pressured environment (when formation pressure is sufficient to force hydrocarbons to the surface at potentially explosive rates) and reduced environmental degradation.
After deviating a bore hole from the vertical, it's obviously no longer completely practical to sustain continuous drilling operations by rotating the drill string in order to rotate the bit. Preferably, only the bit, but not the string, is rotated by a downhole motor attached to the lower end of the string, the motor typically consisting of a rotor-stator to generate torque as drilling fluid passes therethrough, a bent housing to deviate the hole by the required amount and which also encloses a drive shaft therethrough to transmit the rotor/stator's torque to a bearing assembly, and a bit rotatably supported at the downhole end of the bearing assembly for cutting the bore hole.
Electronic means supported by a mule shoe in the bottom hole assembly and connected to the surface by a wire line passing through the interior of the drill string transmits information with respect to the degree and azimuth of the bore hole's trajectory so that it can be plotted and necessary adjustments made. Once the required direction of the hole's trajectory has been attained, the motor must be withdrawn from the well, the bent housing either removed or straightened (if it's of the adjustable sort) and the motor is then run back into the hole to resume drilling operations. Each time the motor requires service, or a change in the hole's trajectory is required, this process must be repeated. This results in substantial costs and down time largely due to the time required to make and break all of the joints as the drill string is tripped in and out of the hole.